I totally get your concern, and chemicals/miscarriages happen, so it’s a worry from the moment you get the faintest line. I still worry now at full term if baby doesn’t move for an hour that something awful has happened. It’s being a mum isn’t it, the worry will never end!
On the pack it says you can take a frer test anytime, but obviously you’re likely to get a stronger line with FMU on any test. Knowing myself as I do I’d probably buy two and take one today and save one for tomorrow, but that’s just me!!